As noted by participants of the SDLU mailing list, the Russian Dial-up User List at the domain dul.ru is no more.
The Russian Dial-up User List website is no longer to be found at dul.ru; when you visit that domain you find a simple Russian-language "this domain is for sale" page.
As of May 19, 2015, this domain seems to have been set to "wildcard" status in DNS. This means that DUL.ru is effectively "listing the world;" any site still using the DUL.ru blacklist zone will reject all inbound mail until this DNSBL is removed from that mail server's configuration.
The Russian Dial-up User list appears to have been a dialup or dynamic blacklist. The intent of this type of anti-spam tool is usually to block SMTP connections from hosts that aren't typically expected to be running mail services.
H/T: Neil Schwartzman